The present invention relates to an air knife, more particularly to an air knife for preventing transfer of liquid from one container to another.
Many industrial processes involve the chemical treatment of articles. Often, this chemical treatment involves two or more steps, each step involving the treatment of the article with a different chemical. When treating discrete articles in such a multistep process, the article to be treated is immersed in a first chemical, removed from the first chemical and dried, and then immersed in a second chemical. The chemicals used are usually liquids.
However, many such multistep processes are applied to elongate articles, or strips of articles, which strips may be separated into discrete articles after the process is complete. A good example of such a process is the electroplating of a strip of components. Such elongate articles or strips of articles are commonly drawn through the first liquid, are dried as they emerge from the first liquid, and are then drawn through the second liquid. This is often a continual process, the drying and immersion in the second liquid taking place on a first portion of the elongate article whilst a portion of the elongate article is still immersed in the first liquid.
An example of such a prior art process is shown in FIG. 1, which shows a transverse cross-sectional view of an air knife according to the prior art.
FIG. 1 shows a wall 1 of a first tank containing a first chemical and a wall 2 of a second tank containing a second chemical, with a space between the two walls 1, 2. The first tank has an aperture 3 in the wall 1 thereof and the second tank has a second aperture 4 in the wall 2 thereof, the first aperture 3 being generally opposed to the second aperture 4. Each aperture 3, 4 has a rubber seal 5 therein, each rubber seal 5 comprising a slit 6 through which an elongate article 7 may pass. Each aperture 3, 4 is positioned above the level of liquid in the tanks.
As shown in FIG. 1, the elongate article 7 is drawn through the first tank in the direction, shown by the arrow 8, out of the slit 6 in the rubber seal 5, contained in the first aperture 3, across the space between the two tank walls 1, 2, through the slit 6 in the rubber seal 5, within the second aperture 4, and into the second tank. Within the space between the walls 1, 2 is positioned air blowing means, such as fans, on either side of the elongate workpiece 7 passing through the space between the tanks. The air blowing means is positioned so as to blow air onto both sides of the elongate article 7 passing between the tanks 1 and 2. Thus, when the elongate article 7 emerges from the first tank, through the slit 6 in the rubber seal 5 of the first aperture 3, the rubber seal 5 in the first aperture 3 will tend to scrape liquid off the elongate article 7. The air, blown by the air blowing means positioned within the space between the walls 1, 2, is intended to blow any remaining liquid off the elongate articles 7. Should the air not blow all remaining liquid off the elongate article 7 the rubber seal 5 in the second aperture 4 is intended to, when the elongate article 7 passes through the slit 6 therein, scrape any residual liquid off the elongate article 7. The rubber seals 5 in the first and second apertures 3, 4 will also tend to prevent egress of liquid from the respective tanks into the space between the walls 1, 2.
This prior art arrangement is known as an air knife, although, of course, any suitable relatively inert gas may be blown at the elongate article 7.
The problem with the air knife currently used is that, despite the rubber seal 5 in the first aperture 3, a relatively large quantity of liquid is still carried by the elongate article 7 out of the first tank. Although much of this liquid is blown off by the air, blown at the elongate article 7 in the space between the walls 1, 2, there is still effective loss of liquid from the first tank 1 and carry over into the tank 2. Thus, liquid carried on the elongate article 7 from the first tank which is not totally blown off by the air or scraped off by the rubber seal 5 in the second aperture 4 may be carried over to the liquid of the second tank.
Another problem is that the air blown onto the elongate article 7 may not, if the liquid in the first tank is a solution, blow all the liquid off the elongate article 7, but will merely dry the solution leaving a thin layer of salt upon the elongate article 7.
It has been estimated by the present inventors that in processes such as the electroplating of components, where gold- or silver-containing liquids are used in the first tank, there can be a 10 to 20% excess of gold or silver dragged over from the first tank, which gold or silver is then either blown or scraped off in the space between the walls 1, 2 or is pulled into the second tank with the elongate article 7. This gold or silver lost from the first tank can be reclaimed by expensive and time-consuming methods, but such methods are so expensive that they are often more costly than the gold or silver which they are intended to reclaim. Accordingly, there is a considerable loss of expensive metals when such a process is used for electroplating a strip of components.
A further problem of the prior art air knife is that there is a considerable loss of space due to the requirement that the first and second tanks must be spaced apart to allow for the positioning of the air blowing means, forming the air knife, therebetween.